First off, you need to find out which train station you're leaving from. TGV's depart from different stations, depending on your destination.

That said, if you're leaving from Gare de Lyon (Lyon, Geneva, many other points south), you can take the Air France bus for Montparnasse -- it leaves every half-hour and the first stop is Gare de Lyon.

For any other station, unless you feel like doing lots of schlepping, spend the US$30-40 for a taxi.